| 1. |
- Jayawickrama, Thamali Rajika, et al.
(author)
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Effect of Stefan flow on drag coefficient of reactive spherical particles in gas flow
- 2018
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In: Turbulence Heat And Mass Transfer 9 (THMT-18). - : Begell House. ; , s. 1089-1092, s. 1089-1092
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Conference paper (peer-reviewed)abstract
- Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle or deposition at the surface can generate additional flow phenomena that affect the heat, mass and momentum transfer between the particle and bulk flow. This work aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow. Fully resolved 3D simulations were carried out for particle Reynolds numbers based on the free stream velocity ranging from 0.5 to 3. Simulations are carried out in foam-extend CFD software, using the Immersed Boundary(IB) method for treating fluid-solid interactions. The simulations were validated against data for particles without reactive flow, and against the analytical solution for Stefan flow around a particle in a quiescent fluid. We found that in the considered range of Reynolds number the drag coefficient decreases linearly with in increase in Stefan flow velocity.
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| 2. |
- Jayawickrama, Thamali Rajika, et al.
(author)
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On the inaccuracies of point-particle approach for char conversion modeling
- 2024
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In: Fuel. - : Elsevier Ltd. - 0016-2361 .- 1873-7153. ; 370
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Journal article (peer-reviewed)abstract
- Char conversion is a complex phenomenon that involves not only heterogeneous reactions but also external and internal heat and mass transfer. Reactor-scale simulations often use a point-particle approach (PP approach) as sub-models for char conversion because of its low computational cost. Despite a number of simplifications involved in the PP approach, there are very few studies that systematically investigate the inaccuracies of the PP approach. This study aims to compare and identify when and why the PP approach deviates from resolved-particle simulations (RP approach). Simulations have been carried out for CO2 gasification of a char particle under zone II conditions (i.e., pore diffusion control) using both PP and RP approaches. Results showed significant deviations between the two approaches for the effectiveness factor, gas compositions, particle temperature, and particle diameter. The most significant sources of inaccuracies in the PP approach are negligence of the non-uniform temperature inside the particle and the inability to accurately model external heat transfer. Under the conditions with low effectiveness factors, the errors of intra-particle processes were dominant while the errors of external processes became dominant when effectiveness factors were close to unity. Because it assumes uniform internal temperature, the models applying the PP approach always predict higher effectiveness factors than the RP approach, despite its accurate estimation of intra-particle mass diffusion effects. As a consequence, the PP approach failed to predict the particle size changes accurately. Meanwhile, no conventional term for external heat transfer could explain the inaccuracy, indicating the importance of other sources of errors such as 2D/3D asymmetry or penetration of external flows inside the particles.
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| 3. |
- Jayawickrama, Thamali Rajika
(author)
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Particle-fluid interactions under heterogeneous reactions
- 2020
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Licentiate thesis (other academic/artistic)abstract
- Particle-laden flows involve in many energy and industrial processes within a wide scale range. Solid fuel combustion and gasication, drying and catalytic cracking are some of the examples. It is vital to have a better understanding of the phenomena inside the reactors involving in particle-laden flows for process improvements and design. Computational fluid dynamics (CFD) can be a robust tool for these studies with its advantage over experimental methods. The large variation of length scales (101- 10-9 m) and time scales (days-microseconds) is a barrier to execute detailed simulations for large scale reactors. Current state-of-the-art is to use models to bridge the gap between small scales and large scales. Therefore, the accuracy of the models is key to better predictions in large scale simulations. Particle-laden flows have complexities due to many reasons. One of the main challenge is to describe how the particle-fluid interaction varies when the particles are reacting. Particle and the fluid interact through mass, momentum and heat exchange. Mass, momentum and heat exchange is presented by the Sherwood number (Sh), drag coefficient (CD) and Nusselt number (Nu) in fluid dynamics. Currently available models do not take into account for the effects of net gas flow generated by heterogeneous chemical reactions. Therefore, the aim of this research is to propose new models for CD and Nu based on the flow and temperature fields estimated by particle-resolved direct numerical simulations (PR-DNS). Models have been developed based on physical interpretation with only one fitting parameter, which is related to the relationship between Reynolds number and the boundary layer thickness. The developed models were compared with the simulation results solving intra-particle flow under char gasification. The drawbacks of models were identied and improvements were proposed. The models developed in this work can be used for the better prediction of flow dynamics in large scale simulations in contrast to the classical models which do not consider the effect of heterogeneous reactions. Better predictions will assist the design of industrial processes involving reactive particle-laden flows and make them highly effcient and low energy-intensive.
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| 4. |
- Jayawickrama, Thamali Rajika
(author)
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Particle-fluid interactions under heterogeneous reactions
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Particle-fluid flows are involved in many natural processes and industrial applications; some examples are drying, solid fuel combustion, gasification, and catalytic cracking. It is vital to understand the phenomena involved in particle-fluid flows in depth for design, predictions and process improvements. Computational fluid dynamics (CFD) can be a robust tool for these studies that complements costly experimental trials. Current computational power and resources do not allow numerical simulations to resolve all physical and chemical scales in a single simulation. State of-the-art in large-scale numerical simulations is to carry out simulations at larger scales with sub-grid models for small-scale phenomena. Therefore, the accuracy of the models is key to better predictions in large-scale simulations.Particle-fluid flows have complexities due to many reasons. One of the main challenges is to describe how the particle-fluid interactions vary when the particles are reacting. Particles and the fluid interact through momentum, heat, and mass exchange. Momentum, heat, and mass exchange are presented by the drag coefficient (Cd), Nusselt number (Nu), and Sherwood number (Sh) in fluid dynamics. Conventional models neglect the effects of net fluid flow generated by heterogeneous chemical reactions called Stefan flow.This work aims to study how Stefan flow affects the momentum, heat, and mass transfer between particles and fluid in a particle-fluid flow. A series of numerical simulations were performed by increasing complexity step by step. Particle boundary layers were resolved in all the simulations, and the particle interior was also resolved in the last stage. With a special interest in entrained flow biomass gasification (EFBG), this work has chosen parameters relevant to EFBG.In the first step, particle-resolved numerical simulations were carried out for an isolated particle immersed in a uniform, isothermal (and non-isothermal) bulk fluid with a uniform Stefan flow. Both isothermal and non-isothermal simulations have shown that the Stefan flow has significant effects on drag coefficient (Cd) and Nusselt number (Nu). We have observed from isothermal results that the decrease/increase of the drag coefficient (Cd) is due to expansion/shrinkage of the boundary layer thickness, which leads to a change in the viscous force. Based on that, a physics based drag coefficient (Cd) model was developed. For the next step, the drag coefficient (Cd) model was extended and modified for a uniform non-isothermal bulk fluid flow. Furthermore, a new Nusselt number (Nu) model was developed using volume-averaged temperature, which captures the variation of thermo-physical parameters due to the temperature gradient between particle and bulk fluid. The model agrees well with the simulation data with a single fitting parameter.The second step was to explore the effects of neighboring particles on the drag coefficient (Cd) with a uniform Stefan flow under isothermal conditions. Stefan flow and neighbor particle effects act on the particle independently when particle distance is greater than 2.5 diameters (L/D > 2.5). However, at L/D ≤ 2.5, Stefan flow effects dominate, and a strong force that expels particles from each other was observed. The models previously developed under ideal conditions (uniform Stefan flow, atmospheric pressure) might not represent realistic conditions at reacting flows. Therefore, the last step of this thesis was particle interior resolved numerical simulations for an isolated char particle under gasifying conditions. The drag coefficient (Cd), Nusselt number (Nu) and Sherwood number (Sh) from the simulations have been compared with conventional models without Stefan flow. We have observed that conventional drag coefficient (Cd) and Nusselt number (Nu) models do not accurately predict the force acting on a particle and heat transfer between the particle and bulk fluid.The performance of the point-particle approach for reacting particle-fluid flows, commonly used in large-scale simulation, was also investigated by comparing it with particle interior resolved simulations for a gasifying particle. The results showed a significant deviation between the results of the point particle model and resolved particle simulations. Several key uncertainties in the models, such as the effectiveness factor and external heat and mass transfer, were identified.This work has shown that the effects of Stefan flow are not negligible in reacting particle-fluid flows. Developed drag coefficient (Cd) and Nusselt number (Nu) models can be used to improve large-scale simulations’ predictions. The study also contributes to widening the understanding physics of particle-fluid interactions in reacting particle-fluid flows. Conventional models for drag coefficient (Cd) and Nusselt number (Nu) (and Sherwood number (Sh)) do not represent the momentum and heat transfer (and mass transfer) between a particle and the bulk fluid accurately when there is a Stefan flow due to heterogeneous reactions during char gasification. Therefore, the models should be further improved considering the effects of Stefan flow.The models developed in this work are idealized for a uniform Stefan flow, atmospheric pressure, and spherical particle. It could be further improved for non-uniform Stefan flow, high pressure, and different geometries. This study mainly focused on the parameter range of gasification for model development. Therefore, it is important to test the effects of Stefan flow for a wider range applicable to other applications, such as combustion, and test whether the phenomena are the same as observed in this work. We focused on char gasification to study the effects of Stefan flow in more realistic conditions and to compare it with the point-particle method. That also could be studied for a wider range of applications and find at what conditions one has to consider the effects of Stefan flow on drag coefficient (Cd), Nusselt number (Nu), and Sherwood number (Sh). Furthermore, it would be important to find the models predicting closer to the resolved-particle simulations for a particle with Stefan flow to be used in the point-particle approach. Improving effectiveness factor models, including non-uniform temperature inside the particle, is also vital.
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| 5. |
- Jayawickrama, Thamali Rajika, et al.
(author)
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The effect of Stefan flow on Nusselt number and drag coefficient of spherical particles in non-isothermal gas flow
- 2021
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In: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 140
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Journal article (peer-reviewed)abstract
- A Stefan flow can be generated during a phase change or reactions of a particle immersed in a fluid. This study investigates the effect of Stefan flow on the exchange of momentum (drag coefficient (CD)) and heat transfer (Nusselt number (Nu)) between the particle and bulk-fluid. Fully resolved simulations were carried out for a flow near a spherical particle immersed in a uniform bulk flow. The immersed boundary method is used for implementing fluid-solid interactions and the particle is considered as a static boundary with fixed boundary conditions. In a non-isothermal flow, the changes in thermophysical properties at the boundary layer played a role in the variation of CD and Nu by a Stefan flow further. The previously developed model for the drag coefficient of a spherical particle in a uniform isothermal flow was modified for a uniform non-isothermal flow. The model is developed based on physical interpretation. A new model is developed for the Nusselt number for a spherical particle with a uniform Stefan flow combining available models in literature. The models are validated for Stefan Reynolds number −8⩽Resf,p⩽25 and particle Reynolds number of 2⩽Ref⩽30 in gas flow (i.e. Pr≈0.7).
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| 6. |
- Jayawickrama, Thamali Rajika, et al.
(author)
-
The effect of Stefan flow on the drag coefficient of spherical particles in a gas flow
- 2019
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In: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 117, s. 130-137
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Journal article (peer-reviewed)abstract
- Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle can generate a Stefan flow that affects heat, mass and momentum transfer between the particle and the bulk flow. This study aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow under isothermal conditions. Fully resolved simulations were carried out for particle Reynolds numbers ranging from 0.2 to 14 and Stefan flow Reynolds numbers from (-1) to 3, using the immersed boundary method for treating fluid-solid interactions. Results showed that the drag coefficient decreased with an increase of the outward Stefan flow. The main reason was the change in viscous force by the expansion of the boundary layer surrounding the particle. A simple model was developed based on this physical interpretation. With only one fitting parameter, the performance of the model to describe the simulation data were comparable to previous empirical models.
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| 7. |
- Jayawickrama, Thamali Rajika, et al.
(author)
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The effects of Stefan flow on the flow surrounding two closely spaced particles
- 2023
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In: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 166
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Journal article (peer-reviewed)abstract
- The aim of the work was to study the effects of neighboring particles with uniform Stefan flow in particle–fluid flows. Particle-resolved numerical simulations were carried out for particles emitting a uniform Stefan flow into the bulk fluid. The bulk fluid was uniform and isothermal. The Stefan flow volume emitted from the two particles is equal, such that it represents idealized conditions of reacting particles. Particles were located in tandem arrangement and particle distances were varied between 1.1 and 10 particle diameters (). Three particle Reynolds numbers were considered during the simulations ( and 14), which is similar to our previous studies. Three Stefan flow velocities were also considered during simulations to represent inward, outward, and no Stefan flow. The drag coefficient of the particles without Stefan flow showed that the results fit with previous studies on neighbor particle effects. When the particle distance is greater than 2.5 diameters (), the effects of Stefan flow and neighboring particles are independent of each other. I.e. an outward Stefan flow decreases the drag coefficient () while an inward Stefan flow increases it and the upstream particle experience a higher than the downstream particle. When , the effect of Stefan flow is dominant, such that equal and opposite pressure forces act on the particles, resulting in a repelling force between the two neighboring particles. The pressure force showed a large increase compared to the viscous force at these distances. The effect of Stefan flow is weakened at higher Reynolds numbers. A model was developed for the calculation of the drag coefficient. The model, which reproduce the results from the numerical simulations presented above, is a product of independent models that describe the effects of both neighboring particles and two distinguished effects of the Stefan flow.
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